MEGATREMA STOKESII. _J. E. Gray._ (sine descript. aut figurâ) Annals
of Philosophy, (new series), vol. 10, Aug. 1825.
_Shell moderately conical, pale-purplish red; orifice oval: basis not
permeated by pores, deeply imbedded in the coral: scutum and tergum
sub-triangular._
_Hab._--Imbedded in the Mycedia (Agaricia) agaricites; therefore from
the West Indies;[115] Brit. Mus. and Stutchbury.
[115] I am greatly indebted to Mr. Dana for having named for me the
coral in which this species was imbedded, and informing me that it
is a West Indian species.
This species comes so close to the last, that I am not sure that I have
acted rightly in retaining it, but I think that it is distinct; and in
this case, it is the representative, on the other side of the Atlantic,
of _P. Anglicum_ of our own side. It will be sufficient to point out
the few points of difference. The shell is much more depressed, with
the orifice oval, larger, and not so narrow. It is apparently of a
paler red, and the radiating ribs perhaps not so prominent. The basis
offers the most important difference, being deeply imbedded in the
coral; and there is not the least appearance of the thin shelly layer,
of which it is composed, being permeated by pores, as, we have seen,
is always the case with _P. Anglicum_. As in this latter species,
the sheath here depends freely. The opercular valves are closely
similar; but in the scutum, the adductor ridge occupies a rather more
central position; and in the tergum, the basal margin is more inclined
towards, or forms a greater angle with, the spur: these differences, by
themselves, I consider quite insufficient to characterise a species;
but conjoined with the flatter shell, the larger orifice, the more
deeply imbedded and non-porose basis, they may, I think, be admitted as
specific. In dimensions, this species seems to attain a slightly larger
size than _P. Anglicum_, for several specimens were .22 of an inch in
diameter.